Integrated circuits (IC), especially analog integrated circuits, need precise, temperature-stable voltage and/or current sources that are processed independently. Traditionally, very precise voltage sources can be produced, e.g., using bandpass or buried Zener circuitry. However, precise current sources that exhibit both process stability and temperature stability are more difficult to manufacture on-chip partially due to the lack of precision resistive components in most IC processing.
Available resistive components used in conventional IC processing have very large temperature coefficients, e.g., measured in the 1000's of ppm/° C., and large process tolerances, e.g., ±30 percent. Accordingly, heretofore, ICs requiring a precision current source have had to rely on external, i.e., off-chip, reference resistance in combination with on-chip voltage reference.
Existing methods of providing precise, on-chip current sources rely on either using a very accurate, resistive material, e.g., thin films of chromium-based metals, and/or combining lower-accuracy solid-state devices in such a way as to provide a final device with a high-degree of accuracy, which is to say, with a low temperature coefficient (TC) and a tight tolerance.
Establishing a process with a very accurate, resistive material, however, requires additional, expensive processing, typically involving additional process masks and fabrication steps. Combining lower-accuracy devices to produce a higher-accuracy device requires testing due to the electrical characteristics of the opposing TC poly-materials, which do not necessarily track each other due to manufacturing tolerances, and, further, requires trimming of the silicon wafer or the resulting, packaged device at multiple temperatures.
Combining or mixing positive TC current sources and negative TC current sources to provide a zero or near-zero TC current source is known in the art. However, verification of the proper “resistance mixture” to achieve the desired zero or near-zero TC mix without having to trim any “over temperature” remains problematic.
Therefore, it would be desirable to provide devices and systems that use readily-available, lower-accuracy solid-state components and standard IC processes to provide a repeatable, precision, zero or near-zero TC, poly-silicon resistance network that provides an optimal “resistance mixture” of opposing TC poly-materials without requiring undue “over temperature” trimming. More particularly, it would be desirable to combine or to mix opposing TC poly-materials having uniform/linear temperature coefficients of resistance and identical thermal mass and thermal conductivity properties.
It would be further desirable to include the devices on-chip as current sources for any IC requiring a precision resistance or a precision current. More specifically, it would be desirable to provide a precision current source to enable power over the Ethernet applications.